This invention relates to a method of rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product.
It is known to produce natural gas from underground reservoirs. The natural gas is often contaminated with nitrogen. The nitrogen may be in part or totally naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation.
U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange relationship with a region, usually the bottom, of the lower pressure rectification column.
In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane at elevated pressure is cooled to a temperature suitable for its separation by rectification. The stream is at least partially liquefied by passage through a condenser-reboiler associated with the bottom of the lower pressure rectification column. A part of the bottom liquid fraction separated in this column is therefore reboiled to provide an upward flow of vapour through the column. Remaining liquid is employed as a feed to the lower pressure rectification column in which a relatively pure liquid methane product is separated as the bottom fraction. A stream of the resulting liquid methane is withdrawn from the lower pressure column and is raised in pressure by a pump. It is warmed by heat exchange to approximately ambient temperature and is thus vaporised.
In order to enhance the liquid nitrogen reflux available to the double rectification column, a stream of nitrogen is withdrawn from the top of the lower pressure rectification column. This nitrogen stream is compressed in a compressor to the operating pressure of the higher pressure rectification column and then combined with a nitrogen stream flowing from the top of the higher pressure rectification column to the condenser-reboiler thermally linking the lower pressure rectification column thereto. As a result, the compressed nitrogen is condensed.
A part of the condensate is returned to the lower pressure rectification column and another part to the higher pressure rectification column. The flow of fluid from the lower pressure rectification to the higher pressure rectification column via the compressor acts as a heat pump. A heat exchanger is incorporated in the heat pump so as to remove heat of compression. This heat exchanger is separate from those in which the pumped product methane stream is warmed.
The vaporisation of the product liquid methane stream in heat exchange with the incoming feed gas stream tends to enhance the thermodynamic inefficiency of the separation method. It is an aim of the present invention to provide a method which makes possible the separation of the feed gas mixture with reduced thermodynamic inefficiency.
According to the present invention, there is provided a method of rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, comprising cooling the feed gas stream in a main heat exchanger, rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, recycling a flow of gas from the lower pressure rectification column to the higher pressure rectification column, a first part of the recycle gas flow being compressed, cooled, liquefied in the condenser-reboiler, and introduced at least in part into the higher pressure rectification column, withdrawing a product methane stream in liquid state from the lower pressure rectification column, raising the pressure of the liquid product methane stream, and vaporising the liquid product methane stream at least partly in the main heat exchanger, wherein a second part of the recycle gas flow is compressed, is cooled, is liquefied in the main heat exchanger or by expansion, and is introduced into the double rectification column in liquid state, and at least part of the cooling of both parts of the recycle gas is performed in the main heat exchanger.
If its pressure is a sub-critical pressure, the second part of the recycle flow is preferably liquefied in the main heat exchanger. If its pressure is at or above the critical pressure, the second part of the recycle flow is preferably cooled in the main heat exchanger to sufficiently low a temperature that it is liquefied on being reduced in pressure by expansion to the operating pressure of the higher pressure rectification column.
By performing the vaporisation of the product methane and the cooling of the recycle stream in the main heat exchanger, the temperaturexe2x80x94enthalpy profile of the streams being warmed in the heat exchanger may be kept closer to that of the streams being cooled therein than in the prior process described in U.S. Pat. No. 4,415,345. As a result, thermodynamic inefficiency is reduced.
Preferably, a vent stream is taken from the recycle gas flow upstream of its compression and is vented according to the invention. Preferably, there is a flow control valve or other means operable to control the size of the recycle stream that is compressed.
There are a number of different options for forming the first and second parts of the recycle flow. In a first arrangement, all the recycle gas is compressed to the same pressure in the same compressor. The recycle gas is partially liquefied, typically by expansion from a supercritical pressure, and a combined flow of liquid and residual vapour is introduced into a top region of the higher pressure rectification column. In a second arrangement, a single, plural stage, compressor is employed. The first part of the recycle flow passes through each stage of the compressor, while the second part of the recycle flow is taken from an upstream stage of the compressor. This second arrangement has the advantage of reducing the work of compressing the recycle gas in comparison with the first arrangement. It is also possible to use separate compressors to compress the first and second parts of the recycle flow.
The second part of the recycle flow is preferably all introduced into the higher pressure rectification column downstream of its liquefaction.
Preferably, for optimum thermodynamic efficiency, a part of the incoming feed gas stream is liquefied in the main heat exchanger. If desired, the resulting partially liquefied feed stream may be subjected to phase separation, the resulting vapour phase being introduced into a bottom region of the higher pressure rectification column and at least part of the liquid phase being introduced into an intermediate mass exchange region of the higher pressure rectification. Preferably, the remainder of the liquid phase is introduced into the lower pressure rectification column at an intermediate mass exchange region thereof. Alternatively, all the liquid phase may be passed into an intermediate mass exchange region of the higher pressure rectification column and a stream of liquid withdrawn from an intermediate region of the higher pressure rectification column and introduced into the lower pressure rectification column.
The pressurised liquid product methane stream is preferably warmed, without being vaporised, in a further heat exchanger upstream of its vaporisation in the main heat exchanger.
Preferably, all the bottom fraction separated in the higher pressure rectification column is withdrawn therefrom and is sent to the lower pressure rectification column. There is therefore no reboiling of this fraction in the higher pressure rectification column.